CN114976827A

CN114976827A - System and method for isolating gain element in laser system

Info

Publication number: CN114976827A
Application number: CN202210526374.1A
Authority: CN
Inventors: 陶业争; R·罗基斯基; D·J·W·布朗; D·J·格里斯; M·卡特斯; J·T·斯特瓦特
Original assignee: ASML Holding NV
Current assignee: ASML Holding NV
Priority date: 2014-12-05
Filing date: 2015-11-06
Publication date: 2022-08-30
Also published as: US20160165709A1; CN107003550B; WO2016089549A1; KR102647219B1; CN107003550A; KR20230058193A; TWI698677B; KR20170094260A; JP6990582B2; JP2021168423A; KR102527174B1; TW201631361A; JP2018506164A

Abstract

Methods and apparatus for protecting a seed laser in a Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) light system are disclosed. An isolation stage positioned on the optical path diverts light reflected from further components in the LPPEUV optical system from reaching the seed laser. The isolation stage includes two AOMs separated by a delay line. The AOM directs light onto the optical path when on and away from the optical path when off. The delay introduced by the delay line is determined so that the opening and closing of the AOM can be timed to direct the forward moving pulse onto the optical path and to divert the reflected light at other times. An isolation stage may be positioned between the gain elements to prevent the amplified reflected light from reaching the seed laser and other potentially harmful effects.

Description

System and method for isolating gain element in laser system

The present application is a divisional application of the invention patent application having an international filing date of 2015, 11/6, international application number PCT/US2015/059573, a chinese national phase entry at 2017, 5/25, chinese national application number 201580064107.5, entitled "system and method for isolating gain element in laser system".

Technical Field

The present application relates generally to Laser Produced Plasma (LPP) Extreme Ultraviolet (EUV) light sources, and more particularly to methods and systems for preventing feedback through gain elements within such light sources.

Background

The semiconductor industry continues to develop photolithographic techniques that are capable of printing ever smaller integrated circuit dimensions. Extreme ultraviolet ("EUV") light (also sometimes referred to as soft x-rays) is generally defined as electromagnetic radiation having a wavelength between 6 nanometers (nm) and 50 nm. EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 5nm to 7nm and is used to produce very small features, for example sub-10 nm features, in substrates such as silicon wafers. To be commercially useful, it is desirable that these systems be highly reliable and provide cost-effective throughput and reasonable process latitude.

Methods of generating EUV light include, but are not necessarily limited to, converting a material into a plasma state with one or more elements (e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc.) using one or more emission lines in the EUV range. In one such method, the desired plasma, often referred to as laser produced plasma ("LPP"), may be produced by irradiating a target material, such as a droplet, stream or cluster of material having a desired line emitting element, with a laser beam at an irradiation site. The line emitting element may be in pure form or in the form of an alloy, such as an alloy that is liquid at the desired temperature, or may be mixed or dispersed with another material, such as a liquid.

In some prior art LPP systems, the droplets in a stream of droplets are irradiated with separate laser pulses to form a plasma from each droplet. Alternatively, some prior art systems are disclosed in which each droplet is irradiated sequentially with more than one light pulse. In some cases, each droplet may be exposed to a so-called "pre-pulse" to heat, expand, vaporize, evaporate and/or ionize and/or generate a weak plasma of the target material, followed by a so-called "main pulse" to generate a strong plasma and convert most or all of the material affected by the pre-pulse into a plasma and thereby produce an EUV light emission. It will be appreciated that more than one pre-pulse may be used and more than one main pulse may be used, and that the functions of the pre-pulse and the main pulse may overlap to some extent.

Since EUV output power in LPP systems is generally scaled in proportion to the drive laser power irradiating the target material, it may also be considered desirable in some cases to employ an arrangement comprising a relatively low power oscillator or "seed laser" and one or more amplifiers for amplifying pulses from the seed laser. The use of a large amplifier allows the use of a seed laser while still providing the relatively high power pulses used in the LPP process.

However, the irradiation of the droplet by the laser pulse may cause reflection and thus light propagating back through the gain element towards the seed laser. This can cause undesirable modulation of the forward laser pulse, and gain loss in the preamplifier. Furthermore, the seed laser may include sensitive optics, and since the pulses from the seed laser have been amplified, the backward propagating light may be of sufficient intensity to damage the relatively fragile seed laser.

For example, in some cases, the amplifier may have a 100,000 (i.e., 10) output ⁵ ) Signal gain of an order of magnitude. In such cases, typical protection devices of the prior art, such as polarization discriminating optical isolators that may, for example, block approximately 93% to 99% of the backward propagating light, may not be sufficient to protect the seed laser from damage.

Accordingly, it is desirable to have an improved system and method for isolating gain elements and protecting seed lasers in such EUV light sources.

Disclosure of Invention

As described herein, AOMs are used to provide isolation between a series of preamplifiers by adding a time delay between pairs of AOMs.

According to some embodiments, a system comprises: the laser seed module is used for generating laser on the light path; a first gain element positioned along the optical path; a second gain element positioned along the optical path after the first gain element; and an isolation stage positioned along the optical path between the first gain element and the second gain element, the isolation stage configured to divert light reflected back along the optical path from the second gain element, the isolation stage comprising: a first acousto-optic modulator (AOM) configured to transition within a first time period between a first state in which light is directed along an optical path and a second state in which light is not directed along the optical path; a second AOM configured to transition within a period of time between a first state in which light is directed along the optical path and a second state in which light is not directed along the optical path, the transition of the second AOM occurring after a time delay; and a delay device positioned between the first AOM and the second AOM and configured to delay transmission of the laser beam between the first AOM and the second AOM by a time selected based on a time period of transition between the two first states and the two second states and a predetermined time period over which both the first AOM and the second AOM remain in the first state.

According to some embodiments, a method comprises: generating laser light on the optical path; passing laser pulses generated from a laser through a first gain element positioned along an optical path; passing the laser pulses through an isolation stage positioned along the optical path between the first gain element and the second gain element, the isolation stage configured to divert light reflected back along the optical path from the second gain element, the isolation stage comprising: a first acousto-optic modulator (AOM) configured to transition over a period of time between a first state in which light is directed along an optical path and a second state in which light is not directed along the optical path; a second AOM configured to transition within a period of time between a first state in which light is directed along the optical path and a second state in which light is not directed along the optical path, the transition occurring after a time delay; and a delay device positioned between the first AOM and the second AOM and configured to delay transmission of the laser beam between the first AOM and the second AOM by a time selected based on a time period for transitioning between the two first states and the two second states and a time period for which both the first AOM and the second AOM remain in the first state; and passing the laser pulses through a second gain element positioned along the optical path after the first gain element.

Drawings

FIG. 1 is a diagram of some components of an embodiment of an LPP EUV system.

FIG. 2 is a diagram of some components of one embodiment of a seed laser module that may be used in an LPP EUV system.

FIG. 3 is a simplified block diagram of one embodiment of a pulse generation system using a seed laser module.

Fig. 4A-4E are simplified block diagrams of one embodiment of an acousto-optic modulator.

Fig. 5A-5B are simplified block diagrams of one embodiment of an isolation stage.

FIG. 6 is a simplified timing diagram depicting how light is diverted through isolation stages in one embodiment.

FIG. 7 is a flow diagram of one embodiment of a method of diverting reflected light.

Detailed description of the invention

In LPP EUV generation systems, a seed laser typically generates seed pulses that are shaped, amplified, and otherwise modified by various elements before irradiating the target material. The seed laser may be fragile and light may reflect from the target material and return to the seed laser. Along the reverse path, reflected light may be added, amplified, and modified by modifying the same elements of the seed pulse. Acousto-optic modulators (AOMs) are therefore typically used as switches to divert or pass light traveling in two directions.

One challenge when using AOMs is that bragg AOMs require a period of time (e.g., one microsecond) to transition from an on state (deflecting light along the optical path) to an off state (turning light from the optical path). This time may be significantly longer than the length of the seed pulse, during which reflected light may pass through the AOM, potentially damaging other elements.

To protect the seed laser and other elements in the LPP EUV system, an isolation stage is positioned between certain elements. The isolation stage includes a delay line positioned between the two AOMs. The AOMs are timed such that each AOM allows a forward propagating pulse generated by the seed laser to pass along the optical path and at other times to divert reflected light from the optical path. When the first AOM deflects the pulse onto the optical path, the second deflects the reflected light, and vice versa. The delay line is used to delay light that has passed through one of the AOMs while the other AOM transitions to a desired state.

FIG. 1 is a simplified schematic diagram of some components of one embodiment of an LPP EUV light source 10. As shown in fig. 1, the EUV light source 10 includes a laser source 12 for generating and transmitting a beam of laser pulses from the laser source 12 along one or more optical paths and into a chamber 14 to irradiate respective targets, such as droplets, at an irradiation region 16. An example of a laser arrangement that may be suitable for use as the laser source 12 in the EUV light source 10 shown in fig. 1 is described in more detail below.

As also shown in FIG. 1, the EUV light source 10 may also include a target material delivery system 26 that, for example, delivers droplets of a target material into the interior of the chamber 14 to the irradiation region 16, where the droplets will interact with one or more laser pulses to ultimately generate plasma and generate EUV emissions. Various target material delivery systems exist in the art and their relative advantages will be apparent to those skilled in the art.

As described above, the target material is an EUV emitting element, which may include, but is not necessarily limited to, a material including tin, lithium, xenon, or a combination thereof. The target material may be in the form of droplets, or alternatively may be solid particles contained within the droplets. For example, elemental tin may be present as pure tin, as SnBr ₄ 、SnBr ₂ 、SnH ₄ And the like as a target material of a tin alloy (e.g., tin-gallium alloy, tin-indium alloy, or tin-indium-gallium alloy, or a combination thereof). Depending on the material used, the target material may be at various temperatures including room temperature or near room temperature (e.g., tin alloys or SnBr) ₄ ) At a temperature above room temperature (e.g., pure tin) or at a temperature below room temperature (e.g., SnH) ₄ ) Is presented to the irradiation zone 16. In some cases, these compounds may be relatively volatile, such as SnBr ₄ . Similar alloys and compounds of EUV emitting elements other than tin and the relative advantages of such materials and those described above will be apparent to those skilled in the art.

Returning to fig. 1, the EUV light source 10 may further comprise an optical element 18, such as a near normal incidence collector mirror having a reflective surface in the form of an oblong ellipsoid (i.e., an ellipse rotated about its major axis), such that the optical element 18 has a first focus within or near the irradiation region 16, and a second focus at a so-called intermediate region 20, where EUV light may be output from the EUV light source 10 and input to a device utilizing EUV light, such as an integrated circuit lithography tool (not shown). As shown in fig. 1, the optical element 18 is formed with an aperture to allow laser pulses generated by the laser source 12 to pass through to the irradiation region 16.

The optical element 18 should have suitable surfaces for collecting and directing EUV light to the intermediate region 20 for subsequent delivery to a device that utilizes EUV light. For example, the optical element 18 may have a graded multilayer coating with alternating layers of molybdenum and silicon, and in some cases one or more high temperature diffusion barrier layers, smoothing layers, capping layers, and/or etch stop layers.

It will be appreciated by those skilled in the art that optical elements other than an elongated ellipsoidal mirror may be used as the optical element 18. For example, the optical element 18 may alternatively be a parabola rotated about its major axis, or may be configured to transmit a light beam having a circular cross-section to an intermediate location. In other embodiments, optical element 18 may utilize coatings and layers other than or in addition to those described herein. Those skilled in the art will be able to select the appropriate shape and composition for the optical element 18 under certain circumstances.

As shown in fig. 1, the EUV light source 10 may comprise a focusing unit 22 comprising one or more optical elements for focusing the laser beam to a focal spot at the irradiation site. The EUV light source 10 may further comprise a beam conditioning unit 24 with one or more optical elements between the laser source 12 and the focusing unit 22 for expanding, manipulating and/or shaping the laser beam and/or shaping the laser pulses. Various focusing units and beam adjusting units are known in the art and may be appropriately selected by those skilled in the art.

As mentioned above, LPP EUV systems in some cases use one or more seed lasers to generate laser pulses that may then be amplified to become a laser beam that irradiates a target material at irradiation site 16 to form a plasma that produces EUV emissions. FIG. 2 is a simplified schematic diagram of one embodiment of a seed laser module 30 that may be used as part of a laser light source in an LPP EUV system.

As illustrated in fig. 2, the seed laser module 30 includes two seed lasers, a pre-pulse seed laser 32 and a main pulse seed laser 34. It will be appreciated by those skilled in the art that in the case of such an embodiment comprising two seed lasers, the target material may be irradiated first with one or more pulses from the pre-pulse seed laser 32 and then with one or more pulses from the main pulse seed laser 34.

The seed laser module 30 is shown with a "folded" arrangement rather than arranging the components in a straight line. In practice, such an arrangement is typical in order to limit the size of the module. To accomplish this, the beams produced by the laser pulses of the pre-pulse seed laser 32 and the main pulse seed laser 32 are directed onto a desired optical path by a plurality of optical components 36. Depending on the particular configuration desired, optical component 36 may be an element such as a lens, filter, prism, mirror, or any other element that may be used to direct a beam of light in a desired direction. In some cases, optical component 36 may also perform other functions, such as altering the polarization of the transmitted light beam.

In the embodiment of fig. 2, the beams from the various sub-lasers are first passed through an electro-optical modulator 38 (EOM). The EOM 38 functions as a pulse shaping unit with the seed laser to shape the pulses generated by the seed laser into pulses having a shorter duration and a faster fall time. Shorter pulse durations and relatively fast fall times can increase EUV output and source efficiency because of the short interaction time between the pulse and the target, and because the unwanted portion of the pulse does not consume amplifier gain. Although two separate pulse shaping units (EOMs 38) are shown, a common pulse shaping unit may alternatively be used to trim both the pre-pulse and the main pulse seed.

The beam from the seed laser is then passed through an isolation stage comprising acousto-optic modulators (AOMs) 40 and 42 and a beam delay device 41. As will be explained below,

AOMs

40 and 42 act as "switches" or "light valves" that operate to divert any reflection of the laser beam from the target material from reaching the seed laser; as noted above, the seed laser typically contains sensitive optics, and AOMs 40 and 42 thus prevent any reflections from causing damage to the seed laser elements. The delay means 41 are such as known in the art; as seen more clearly in the retarding device 48, the retarding device 41 has a beam folding optical arrangement comprising optical components such as mirrors, prisms, etc., so that light passing through the cell travels an optical retardation distance d _Delay (ii) a Using about 3X 10 ⁸ Estimated beam velocity in meters per second, beam delay per meter being light on the optical pathAn additional travel time of approximately 3.33ns is added. Additional details regarding the delay device 41 and the isolation stage are discussed in more detail below (particularly in relation to the first isolation stage 33 of fig. 3). In the embodiment shown here, the beams from the various sub-lasers pass through two AOMs. Further, as will be discussed elsewhere herein, the isolation stage may be located elsewhere in the seed laser module 30.

After passing through AOM 42, the two beams are "combined" by beam combiner 44. Since the pulses from the various sub-lasers are generated at different times, this effectively means that two temporally separated beams are placed on a common optical path 46 for further processing and use.

After being placed on the common optical path, the beams from one of the seed lasers (again, only one at a time) pass through a further beam delay device 48 having a beam folding optical arrangement. Next, the beam is directed through at least one preamplifier 50 and then through a beam expander 52. Thereafter, the beam passes through a thin film polarizer 54 and is then directed upward by an optic 56, the optic 56 again being an element that directs the beam to the next stage in the LPP EUV system and may also perform other functions. From the optical component 56, the beam typically passes to one or more optical amplifiers and other components, as will be explained below.

Various wavelength tunable seed lasers suitable for use as both pre-pulse and main pulse seed lasers are known in the art. For example, in one embodiment, the seed laser may be CO with a sealed fill gas ₂ Laser, sealed fill gas comprising CO at sub-atmospheric pressure (e.g., 0.05 to 0.2 atmospheres) ₂ And pumped by a radio frequency discharge. In some embodiments, a grating may be used to help define the optical cavity of the seed laser, and the grating may be rotated to tune the seed laser to a selected rotation line.

Fig. 3 is a simplified block diagram of one embodiment of a seed pulse generation system 60. Like the seed laser module 30, the seed pulse generation system 60 generates seed pulses, shapes the seed pulses, and amplifies the seed pulses. However, the seed pulse generation system 60 includes two

preamplifiers

74 and 84 in place of one preamplifier 50 of the seed laser module 30 of FIG. 2. The addition of the second pre-amplifier and the additional gain provided by the second pre-amplifier may result in a higher probability that a power amplifier located outside of the seed lasing system 60 will lase itself, inducing modulation of the forward laser pulse and loss of gain to the

pre-amplifiers

74 and 84 in the seed lasing system 60. The spontaneous laser light generated in the power amplifier has been observed as a pulse having a wide duration lasting several microseconds. To attenuate these effects of adding the second preamplifier, the seed pulse generation system 60 of fig. 3 includes an additional isolation stage positioned between the elements of the seed laser module 30 of fig. 2 to prevent reflected light from reaching the seed laser and the second preamplifier. The isolation stage of the seed pulse generation system 60 may be added to or implemented in the seed laser module 30 of fig. 2, as will be apparent to those skilled in the art.

In fig. 3, although the seed laser 62 is depicted as a single unit, it produces a beam as described in relation to the pre-pulse seed laser 32 and the main pulse seed laser 34 of fig. 2. Again, as will be understood by those skilled in the art, the seed pulse generation system 60 may include more than one seed laser 62. The EOM 64 shapes the pulses as described in relation to the EOM 38 of fig. 2 above.

The first isolation stage 66 is positioned between the EOM 64 and the first preamplifier 74. First isolation stage 66 includes a first AOM 68, a delay device 70, and a second AOM 72; the delay device 70 again has a beam folding optical arrangement. The first isolation stage 66, like

AOMs

40 and 42 and delay line 41 of fig. 2, operates to divert any reflections of the laser pulses from the target material from reaching the seed laser 62. As described in further detail herein, the isolation stage 66 provides improved isolation from the amplified pulses that have passed through the first preamplifier 74.

To amplify the seed pulse generated by the seed laser 62, the seed pulse is passed through two or more preamplifiers instead of just one preamplifier as shown in FIG. 2. By using more than one preamplifier, the seed pulse can be amplified in stages, which has a number of benefits. The use of separate amplifiers with smaller individual gains prevents spontaneous lasing of the optical elements. Another benefit derived from the use of an isolation stage with multiple preamplifiers is that the gain after 99% of the reflected light is diverted is so high that intermediate amplification of the reflected light can be diverted before even 1% of the reflected light is still strong enough to damage the seed laser 62.

First preamplifier 74 is followed by a second isolation stage 76 comprising a first AOM 78, a delay device 80, and a second AOM 82. The second isolation stage 76 is capable of diverting reflected light originating from other portions of the LPP EUV system than the first isolation stage. Since the second preamplifier 84 follows the second isolation stage 76 for pulses traveling to the irradiation site, all reflected light reaching the second isolation stage 76 will also have been amplified by the second preamplifier 84.

Although not depicted, a further isolation stage may follow the second preamplifier 84 before the beam is directed to yet further elements of the LPP EUV generating system. Such further isolation stages may divert reflected light arriving from further components in the LPP EUV system before the reflected light is amplified by the second preamplifier 84.

Fig. 4A-4E are simplified block diagrams of one embodiment of an AOM 90, such as those depicted in seed pulse generation system 30 of fig. 2 and 60 in fig. 3. AOM 90 may be a bragg AOM, which is familiar to those skilled in the art and is depicted at five points in time during its operation. As described above with respect to AOMs 40 and 42 of FIG. 2, AOM 90 acts as a "switch" or "light valve" to deflect or steer light depending on its current state. AOM 90 uses the acousto-optic effect, in which acoustic (acoustic) waves within the material cause changes in the optical properties of the material to diffract and shift the frequency of light passing through AOM 90.

As is known in the art, AOM 90 is typically activated by a piezoelectric transducer (PZT) attached to one end of the AOM. Power, typically Radio Frequency (RF) power, is applied to the PZT as an oscillating electrical signal, which causes the PZT to vibrate and create acoustic waves 92 in the AOM. When no power is applied, there is therefore no acoustic wave 92, and the light is transmitted directly through the AOM; when power is applied, an acoustic wave is present and the AOM operates in a "deflection mode" in which an incident beam is deflected onto the beam path and shifted in frequency. The amplitude of the RF power applied to the PZT in the deflection mode is sufficient to deflect the light into the beam path. As will be apparent to those skilled in the art, the amplitude need only direct the light to a sufficient degree for practical deflection. Due to the desired switching speed, power is typically applied to the PZT in the direction of the processor or controller.

As depicted in fig. 4A-4E, acoustic wave 92 travels across AOM 90. The acoustic wave 92 has a known length based on the time period T during which power is applied to the PZT and the velocity V. The AOM 90 is positioned in the optical path to intercept the pulses at a beam aperture 94. The beam aperture 94 is depicted in the figure as a circle having a diameter "d", but is not necessarily a physical feature of the AOM 90. The amount of time T that the acoustic wave 92 overlaps the beam aperture 94 to allow the pulse to pass (referred to as the minimum acoustic packet size) can be calculated from the beam diameter and pulse duration by the following equation:

T＝D/V+dT

where D is the beam diameter, V is, as described above, the velocity of the acoustic wave as it propagates through AOM 90 (constant for AOM), and dT is the optical pulse duration (constant for AOM). When the beam diameter is 4 mm, the velocity of the acoustic packet (acoustic packet) is 5500 m/sec, and the optical pulse duration is 200 ns, the minimum acoustic packet size generated is 927 ns.

Upon initiation as shown in FIG. 4A, the acoustic wave 90 propagates in one direction across AOM 90. When the acoustic wave 90 overlaps the beam aperture 94 of the AOM 90 (as shown in fig. 4C), the beam is deflected onto the optical path to continue to other elements. When the acoustic wave 92 does not overlap the beam aperture 94, light from either direction in the seed generation system 60 is passed so as not to follow the optical path. Thus, when no acoustic wave is present at the beam aperture 94, the reflected light is less likely to reach the seed laser 32, as shown in fig. 4A and 4E.

When the acoustic wave 92 partially overlaps the beam aperture 94 as shown in fig. 4B and 4D, a portion of the light hitting the portion with the acoustic wave 92 is deflected onto the optical path, and the remaining portion passes through the AOM 90. Thus, a portion of the reflected light traveling from the chamber toward the seed pulse generator can pass through the portion where the acoustic wave 92 overlaps the beam aperture 94 and be directed onto the optical path. The remaining portion of the reflected light is prevented from following the optical path in the absence of acoustic waves. In some cases, the deflected portion of the light beam exhibits a phenomenon known as "beam imaging" in which the deflected portion maintains the shape of a portion of the light beam when deflected. Beam imaging is observed as a shift of the beam from the center of the beam aperture 94 and may have a non-circular, oval, or semi-circular shape.

Fig. 5A and 5B are simplified block diagrams of one embodiment of an isolation stage, such as isolation stages 66 and 76. In FIG. 5A, the isolation stage is shown as being comprised of

AOMs

106 and 112 and delay device 110. Fig. 5A and 5B together depict the relative state of the AOM as the seed pulse and reflected light, respectively, pass through the isolation stage. As described above, when the acoustic wave 92 overlaps the beam aperture 94, the light is deflected onto the optical path depicted as optical path 104. When the acoustic waves 92 do not overlap the beam aperture 94, the light is directed away from the optical path 104. As is known in the art, light passes through the AOM when the acoustic wave 92 is not present, however, for simplicity, fig. 5 depicts the light path 104 as a straight line.

As seen in fig. 5A, in operation, a pulse 102 generated by the seed laser 62 reaches the first AOM106 when an acoustic wave 92 propagating across the AOM106 in a direction 108 reaches the beam aperture 94. The pulse 102 is passed along the optical path 104 to the delay device 110. When the pulse 102 passes the AOM106, the second AOM112, which is located immediately after the delay device 110, is in the following state: so that it prevents reflected light originating outside the isolation stage from entering the delay means 110 and proceeding back to the seed laser 62.

The acoustic waves 92 in the first AOM106 and the second AOM112 continue to propagate while the pulse 102 travels through the delay device 110. In the second AOM112, the acoustic wave 92 is generated after the acoustic wave 92 is generated in the first AOM106 such that it is delayed by a predetermined amount of time. The delay between when the acoustic wave is generated and the amount of delay introduced into the optical path by the delay device 110 is coordinated so that when the pulse 102 reaches the second AOM112, the acoustic wave 92 is at the beam aperture 94 and is deflected so as to continue further along the optical path 104.

While the second AOM112 deflects the pulse 102 onto the optical path 104, the first AOM106 is in an opposite state that prevents light from following the optical path 104. Thus, as seen in fig. 5B, if any reflected light 114 passes through the second AOM112 while the second AOM112 directs the forward pulse partially or fully onto the optical path 104, the reflected light 114 continues through the delay device 110 while the acoustic wave 92 in the first AOM106 propagates out of the beam aperture 94. After the acoustic wave 92 exits the beam aperture 94 on the first AOM106, the reflected light 114 is prevented from continuing back to the seed laser on the optical path 104.

Fig. 6 is a timing diagram 600 depicting how reflected light is diverted by isolation stages (e.g., isolation stages 66 and 76). Timing diagram 600 depicts one embodiment of a timing pattern that may be used. Based on the description provided below, those skilled in the art will be able to generate and implement alternative timing patterns to prevent reflected light from reaching the seed module.

As depicted in

graphs

130 and 140, RF power is supplied to the first AOM106 and remains on for a time equal to the sum of the time required for the acoustic wave to cover the beam aperture 94 (labeled TRISE) and the optical pulse duration (labeled TP). After a time delay (labeled TDELAY), RF power is provided to the second AOM112 in

graphs

150 and 160 as described in relation to the first AOM 106.

The delay between times labeled "TP" is the delay introduced by delay device 110. The delay means 110 may for example provide a delay of at least 300 nanoseconds. The timing of the AOM and the amount of delay introduced by the delay line vary depending on the diameter of the beam, the direction of acoustic wave propagation within the AOM, and the presence of beam imaging. The delay may be calculated in a variety of ways for different implementations. The following example implementation is provided as guidance to illustrate how the necessary amount of delay may be determined.

The diameter of the beam affects the amount of time TRISE that the acoustic wave takes to block the beam aperture 94. For a compound having a composition defined as 1/e ² A gaussian beam of size, TRISE can be approximated as the time across its width. As will be apparent to those skilled in the art, the TRISE is 610 nanoseconds for a 2.7 millimeter beam and 1470 nanoseconds for a 6.5 millimeter beam.

When the acoustic waves within the AOMs propagate in the same direction, as discussed in relation to fig. 5, the minimum amount of delay that should be provided by the delay devices positioned between the AOMs in the isolation stage can be calculated as follows:

TDELAY>TRISE+TP/2

where TDELAY is the delay provided by delay device 110, TRISE is the time required for the acoustic wave to block the beam aperture in the AOM, and TP is the optical pulse duration. The delay is at least the calculated time that allows the AOMs to open at different times, and is the time difference between when the respective gates are open long enough to ensure that the two AOMs combined are completely or substantially closed when the reflected light reaches the isolation stage. As will be apparent to those skilled in the art based on this disclosure, the upper limit of the time delay is defined by the properties of the delay device 110, including but not limited to the length, volume, and loss of the delay device 110.

In instances where the corresponding acoustic waves in the AOM propagate in opposite directions, the AOM is said to be cross-fired. Cross-priming of AOMs is accomplished by priming an acoustic wave at a first end in a first AOM and at an opposite end in a second AOM. Because the acoustic waves travel in opposite directions when the AOMs are cross-fired, the minimum amount of delay provided by the delay devices positioned between the AOMs in the isolation stage can be calculated as follows:

TDELAY>(TRISE+TP)/2

in some examples, beam imaging may be observed, as depicted with diagram 170. As explained above, beam imaging may occur when an acoustic wave partially overlaps a beam aperture on the AOM. As depicted in fig. 6, the beam imaging phenomenon may also be utilized to reduce the amount of delay introduced by the delay device such that a first portion of the reflected light is diverted at second AOM112 and the remaining portion of the light is diverted by first AOM 106. Because the AOM need only be partially turned off to divert a portion of the reflected light, the delay introduced by the delay device 110 can be shortened according to the same equation described above for a cross-start AOM.

FIG. 7 is a flow diagram of one embodiment of a method 200 of diverting reflected light using an isolation stage. The operations of method 200 may be performed during overlapping time points as described herein.

In operation 202, laser pulses are optionally passed through a first gain element. The first gain element may be a preamplifier, such as preamplifier 74 of fig. 3.

Next, a first AOM (such as first AOM106 of fig. 5) is transitioned to deliver laser pulses onto an optical path (e.g., optical path 104 in fig. 5) in operation 204. As discussed above, the first AOM is transformed by creating an acoustic wave that propagates across the AOM to overlap with the beam aperture (e.g., beam aperture 94 in fig. 5).

Next, in operation 206, the laser pulses are passed through a delay device (e.g., delay device 110 of fig. 5). The delay device increases an amount of travel time between the first AOM and the second AOM in the isolation stage.

Next, in operation 208, a second AOM (e.g., second AOM112 of fig. 5) is transitioned to deliver the laser pulses onto an optical path (e.g., optical path 104) to an optional second gain element (e.g., preamplifier 84 of fig. 3). The second AOM is similarly transformed as the acoustic wave propagates through the beam aperture in the AOM.

Next, in operation 210, the first AOM is transitioned to divert the reflected light that passed through the second AOM and the retarding device. The first AOM is transformed as the acoustic wave propagates through a beam aperture in the AOM. In practice, operation 210 preferably follows operation 204 and overlaps with

operations

206 and 208.

Next, in operation 212, the second AOM is transitioned to divert reflected light from further components in the LPP EUV system. In operation, operation 212 preferably follows operation 208 and overlaps with operation 210.

The isolation stage described herein allows pulses to travel along an optical path within the seed pulse generation system while preventing reflected light traveling in the opposite direction along the optical path from reaching sensitive and fragile components upstream of the isolation stage. The isolation stage introduces a delay between the two AOMs within the system. The delay can be shortened by cross-starting the AOM or when the phenomenon of beam imaging is observed.

The disclosed method and apparatus have been described above with reference to several embodiments. Other embodiments will be apparent to those skilled in the art in view of this disclosure. Certain aspects of the described methods and apparatus may be readily implemented using configurations other than, or in combination with, those described in the embodiments above. For example, different algorithms and/or logic circuits may be used, perhaps more complex than those described herein, and possibly different types of driven lasers and/or focusing lenses.

Note that as used herein, the term "optical component" and derivatives thereof includes, but is not necessarily limited to, one or more components that reflect and/or transmit and/or manipulate incident light, and includes, but is not limited to, one or more lenses, windows, filters, wedges, prisms, gratings, gradations, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons, and mirrors including multilayer mirrors, near normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors, and combinations thereof. Furthermore, unless otherwise specified, the terms "optical device", "optical component", and derivatives thereof, as used herein, are not meant to be limited to components that advantageously operate alone or in one or more particular wavelength ranges, such as at EUV output light wavelengths, irradiation laser wavelengths, wavelengths suitable for metrology, or some other wavelength.

As noted herein, various variations are possible. In some cases a single seed laser may be used instead of the two seed lasers illustrated in fig. 2. One common isolation stage may protect both seed lasers, or either or both seed lasers may have their own isolation stage for protection. The isolation stage may be located elsewhere in the seed generation system 60, such as after the preamplifier 84. A single bragg AOM may be used in some cases, or more than two bragg AOMs may be used to protect a single seed laser if desired. Other types of AOMs may also be used.

It should also be appreciated that the described methods and apparatus may be implemented in numerous ways, including as a process, an apparatus, or a system. The methods described herein may be implemented by program instructions for instructing a processor to perform such methods, and such instructions are recorded on a computer readable storage medium such as a hard disk drive, a floppy disk, an optical disk such as a Compact Disk (CD) or a Digital Versatile Disk (DVD), a flash memory, etc., or via a computer network, where the program instructions are transmitted over an optical or electronic communication link. Such program instructions may be executed by means of a processor or controller or may be incorporated in a fixed logic element. It should be noted that the order of the steps of the methods described herein may be altered and still be within the scope of the present disclosure.

These and other variations upon the embodiments are intended to be covered by the present disclosure, which is limited only by the appended claims.

Claims

1. A system for preventing feedback through a gain element of a laser beam, the system comprising:

a first gain element positioned along an optical path of the laser beam;

a second gain element positioned along the optical path after the first gain element; and

an isolation stage positioned along the optical path between the first gain element and the second gain element, the isolation stage configured to divert light that has been reflected back along the optical path by the second gain element, the isolation stage comprising:

a first acousto-optic modulator configured to transition between a first state in which light is directed along the optical path and a second state in which light is not directed along the optical path over a first period of time, the first period of time being based on a diameter of the laser beam;

a second acousto-optic modulator configured to transition between a first state in which light is directed along the optical path and a second state in which light is not directed along the optical path within a second time period, the second time period being based on a diameter of the laser beam, and the transition of the second acousto-optic modulator occurring at a time after the transition of the first acousto-optic modulator; and

a delay device positioned between the first acousto-optic modulator and the second acousto-optic modulator and comprising a beam folding optical arrangement comprising optical components configured to delay transmission of light between the first acousto-optic modulator and the second acousto-optic modulator by a time determined based on a first transition time and a second transition time of the acousto-optic modulator such that any light that has passed through the second acousto-optic modulator reflected back along the optical path will not pass through the first acousto-optic modulator,

wherein the delay is further based on the occurrence of beam imaging,

wherein, if beam imaging occurs, the delay is further determined such that a first portion of the laser beam is diverted by the second acousto-optic modulator and the remaining portion of the laser beam is diverted by the first acousto-optic modulator.

2. The system of claim 1, further comprising one or more other elements positioned outside of the second gain element.

3. The system of claim 2, wherein the one or more other elements comprise an Extreme Ultraviolet (EUV) plasma chamber.

4. The system of claim 2, wherein the one or more other elements comprise a power amplifier.

5. The system of claim 1, wherein the first gain element and the second gain element comprise preamplifiers.

6. The system of claim 1, further comprising a second isolation stage positioned outside of the second gain element along the optical path.

7. The system of claim 1, further comprising a second isolation stage positioned along the optical path between the first gain element and a module that generates the laser beam.

8. The system of claim 1, wherein the isolation stage is further configured to prevent spontaneous lasing in the first gain element by diverting reflected light.

9. The system of claim 1, wherein the first acousto-optic modulator and the second acousto-optic modulator are cross-activated.

10. A method for preventing feedback through a gain element of a laser beam, the method comprising:

passing laser pulses generated from a laser beam through a first gain element positioned along an optical path;

passing the laser pulses through an isolation stage positioned along the optical path after the first gain element, the isolation stage configured to divert light reflected back along the optical path from any element located outside of the isolation stage, the isolation stage comprising:

a delay device positioned between the first acousto-optic modulator and the second acousto-optic modulator and comprising a beam folding optical arrangement comprising optical components configured to delay the transmission of light between the first acousto-optic modulator and the second acousto-optic modulator by a time determined based on a first transition time and a second transition time of the acousto-optic modulator such that any light that has passed through the second acousto-optic modulator that is reflected back along the optical path will not pass through the first acousto-optic modulator; and

passing the laser pulse through a second gain element positioned along the optical path after the isolation stage,

wherein the delay is further based on the occurrence of beam imaging,